Ammonia is a highly volatile noxious material with adverse physiological effects, which becomes intolerable at very low concentrations and presents substantial environmental and operating hazards and risk. It is classified as a hazardous material and many precautions are required in transporting and handling it safely. Urea, on the other hand, is a stable, non-volatile, environmentally benign material that is safely transported, stored and handled without such risk and, accordingly, can serve as a safe source of ammonia. The processes of this invention minimize the risks and hazards associated with the transport, storage and use of anhydrous and aqueous ammonia.
Many industrial plants require the supply of large quantities of ammonia, which frequently must be transported through and stored in populated areas. Important users among these are industrial furnaces, incinerators and the electric power generation industry. All of these are faced with a lowering of the amount of nitrogen oxides being discharged to the atmosphere in the combustion gases being emitted from their operations, as required by environmental regulations. Another important use is for the so-called "conditioning" of flue gas by which an improved collection and removal of particulate matter (fly ash) is obtained.
One of the important methods for removing nitrogen oxides derived from the burning of fossil fuels embodies their conversion to inert nitrogen gas by reaction with amine-type reductant materials, by processes such as Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR). Two main reductant materials have achieved commercial acceptance for this purpose, namely, ammonia and urea. Ammonia is superior to urea as such for SNCR in several important aspects for this application and is required for SCR applications but, as previously related, ammonia presents substantial environmental and operating hazards and risk because of its high volatility and noxious nature. Numerous accidents that have resulted in deaths have occurred in the transport and handling of ammonia and local authorities have placed restrictions on its use in many locations. Urea, on the other hand, is a stable, non-volatile, environmentally benign material that is safely transported, stored and handled without such risk.
This invention is particularly useful for supplying ammonia for the SCR and SNCR processes for removal of nitrogen oxide from combustion gases, the conditioning of flue gas for improving the removal of particulate matter, and other applications in which ammonia is used, by which the environmental hazards of transporting and storing anhydrous and aqueous ammonia solutions may be avoided.
In the SNCR method, ammonia, urea or amine-type materials are injected into the hot flue gas stream, usually in-furnace while in a temperature range of 2000.degree. F.-1800.degree. F. In the SCR method, ammonia is the only reductant used, and the reaction is carried out at a lower temperature level, typically 750.degree. F.-600.degree. F., the specific temperature level being controlled by the catalyst system used.
The chemical reaction taking place in the flue gas by which NO and NO.sub.2 are removed is shown below for ammonia and urea:
Ammonia EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O [1] EQU 2NO.sub.2 +4NH.sub.3 +O.sub.2 .fwdarw.3N.sub.2 +6H.sub.2 O [2]
Urea EQU 4NO+2CO(NH.sub.2).sub.2 +O.sub.2 .fwdarw.4N.sub.2 +4H.sub.2 O+2CO.sub.2[ 3] EQU 2NO.sub.2 +2CO(NH.sub.2).sub.2 +O.sub.2 .fwdarw.3N.sub.2 +4H.sub.2 O+2CO.sub.2 [ 4]
Compared to urea, ammonia is more reactive, is more easily dispersed uniformly into the flue gas stream and is active over a broader temperature range, as well as being more effective in efficiency. Urea, as such, while also an effective reductant, forms unwanted byproducts, such as carbon monoxide (CO) and nitrous oxide (N.sub.2 O), both of which are now under critical scrutiny by environmental authorities.
In the application of ammonia for the "conditioning" of flue gas, ammonia forms ammonium bisulfate with the sulfur oxides also present in the flue gas. These deposit and collect on the fine particles to form larger sticky agglomerates of the fly ash particles which makes their removal easier and more effective by both electrostatic collectors and fabric filters.
In this invention urea is converted to ammonia at the site where the combustion gases are being produced and there is no need to transport and store anhydrous ammonia or its aqueous solutions. Urea is the material that is shipped, stored and safely handled. The concept of this invention is also applicable to many other industrial uses of ammonia, such as pH adjustment, minimization of corrosion problems, heat treating of metal, etc.
The basic chemistry employed in the invention is a reverse of that employed in the industrial production of urea from ammonia and carbon dioxide and employs two reaction steps, as follows: ##STR1## The first reaction in which urea hydrolyzes to form ammonium carbamate is mildly exothermic, while the second, in which ammonia and carbon dioxide are produced is strongly endothermic, with the result that the reaction to release ammonia and carbon dioxide requires heat and quickly stops when the supply of heat is withdrawn. Excess water promotes the hydrolysis reaction, the overall reaction for which is as follows: EQU xH.sub.2 O+NH.sub.2 CONH.sub.2 .fwdarw.2NH.sub.3 +CO.sub.2 +x-1H.sub.2 O[7]
In the thermal hydrolysis process of the invention, the liberation of ammonia commences at around 110.degree. C. and becomes rapid at around 150.degree. C. to 160.degree. C., with or without catalytic assistance.
The generation of ammonia from urea is uniquely applicable to the control of nitrogen oxide emissions and the "conditioning" of combustion gas streams. The products of the hydrolysis are not foreign to those in combustion gas. The composition of combustion gas streams will typically have H.sub.2 O concentrations ranging from 7% to 13% and CO.sub.2 concentrations arranging from 6% to 14% with NO concentrations ranging from 20 ppm up to 2000 ppm. Hydrolysis of a 30% urea solution produces an off gas with a composition of 20.5% NH.sub.3, 10.2% CO.sub.2 and 69.3% H.sub.2 O (Molar). Ammonia for both SCR and SNCR systems for NO control is injected at NH.sub.3 :NO ratios ranging from 0.5 to 2.0. With urea hydrolysis produced ammonia, the same NH.sub.3 :NO ratios are required and the accompanying CO.sub.2 and H.sub.2 O will add only a small amount of additional material compared to what is already present in the combustion gas stream, since NO concentrations in flue gas are on the order of 100 times less than their CO.sub.2 and H.sub.2 O content. The CO.sub.2 and H.sub.2 O added in the urea hydrolysis products will have no impact on the operation of the combustion process. There is also no major impact on the NO control system. Typically, ammonia is diluted with air, recycled flue gas, or steam prior to injection to insure a larger gas flow so as to provide a uniform distribution of the ammonia being fed to the flue gas for reaction with the nitrogen oxides. There must be a close stoichiometric matching and intimate mixing and molecular contact of the introduced ammonia with all of the nitrogen oxide molecules within a very short time. Otherwise, there will be either a discharge of unreacted ammonia and/or unreacted oxides of nitrogen in the off-gas. The treatment stream is typically introduced by distribution grids covering the entire cross-sectional flow area or by high velocity injection nozzles.
The prior art relating to the hydrolysis of urea has been mainly concerned with two areas of application: (1) a reduction in the amount of urea remaining in the low concentration waste streams produced in urea manufacture by its conversion back to ammonia and carbon dioxide under low pressure, which are then recovered and dehydrated and recycled back to the urea synthesis step, or (2) for the removal of nitrogen oxides from combustion gas streams by reaction with urea hydrolysates which are produced by a partial hydrolysis under high pressure of a urea solution, during which the ammonia formed is not released as generated and is held in solution while the urea is being heated and partially reacted.
The first of these, in which the urea in the dilute waste water streams discharged by urea manufacturing plants is converted to ammonia and carbon dioxide for recycle has been disclosed by Mavrovic in U.S. Pat. No. 3,826,815, Van Moorsel in U.S. Pat. No. 3,922,222 and Schell, in U.S. Pat. Nos. 4,087,513 and 4,168,299. None disclose the use of urea, as a source of ammonia for other uses, nor as a means of avoiding the hazardous aspects involved in the shipping and employment of ammonia in other industrial operations. In particular, there is no visualization of feeding a solution of urea to a reactor to produce a gaseous stream of ammonia, carbon dioxide and water which is generated at a controlled rate for use for other purposes than urea manufacture. No disclosure was made as to the incorporation of a urea hydrolysis step for producing ammonia for use in removing oxides of nitrogen by SNCR (Selective Non-Catalytic Reduction) or SCR (Selective Catalytic Reduction) processes from waste combustion gas streams, the "conditioning" of flue gas to improve particulate collection, or for other applications in which ammonia is employed. All of these recovery processes involve stripping and distillation devices for removal and concentration of the ammonia formed.
In the second group, von Harpe et. al. in U.S. Pat. No. 5,240,688, disclose a system and operating conditions for the partial hydrolytic decomposition of urea to produce ammonium salts and unreacted urea, all in solution form, that are useful for the reduction of nitrogen oxides in combustion gas streams, but not as a gaseous ammonia-containing product that is free of unreacted urea and ammonium carbamate.
The von Harpe patent is specifically directed to conditions for the SNCR (Selective Non Catalytic Reduction) process for NO.sub.x control. Operating conditions are selected such that hydrolysis products formed will be emitted from the system as ammonium salts, either by operating at pressures in excess of 1,200 pounds per square inch or by cooling the hydrolysis products to below 70.degree. F. There is no suggestion about operating at the lower pressure levels of the present invention in which ammonia and carbon dioxide are emitted in gaseous form and which are also useable by SCR (Selective Catalytic Reduction) process systems and other processes needing ammonia gas.
The process of this invention where an aqueous solution of urea is continuously fed into a heated reactor and a continuous stream of gaseous ammonia, carbon dioxide and water produced and discharged without any urea or ammonium salts for use by both SCR and SNCR nitrogen oxides reduction processes, or for other uses of ammonia, is not disclosed by von Harpe.
Lyon in U.S. Pat. No. 3,900,554 describes a process in which ammonia is used to reduce nitrogen oxide concentrations in combustion exhaust gas streams for use with the so called SNCR (Selective Non-Catalytic Reduction) method. In U.S. Pat. No. 4,220,632, Pence, et. al. describes a process in which ammonia is used to reduce nitrogen oxides in combustion exhaust gas in the presence of a catalyst by the SCR (Selective Catalytic Reduction) process.
Jones, in U.S. Pat. No. 5,281,403, describes a method for removing nitrogen oxides from a combustion gas stream in which a solution of urea is partially hydrolyzed by passage of a urea solution in an injection lance which is heated directly by the hot combustion gas stream containing the nitrogen oxides into which the lance is inserted, while keeping the urea hydrolysis products in the liquid phase in the presence of a conversion catalyst, and then injecting the partially converted urea solution from the lance system into the combustion gas. The Jones patent is directed to the higher temperature (1800-2000.degree. F.) SNCR process and does not produce a gaseous ammonia-containing product stream free of unreacted urea that can be used directly in both the SNCR and the lower temperature SCR processes, as by the present invention. Jones teaches that high pressures of greater than 900 psig are required to keep the ammonia formed in the hydrolysis reaction in solution and that conversions of greater than 90% may be obtainable, whereas the present invention operates effectively at much lower pressures, of only 20 to 500 psig, and preferably at 60 to 150 psig. Ammonia and carbon dioxide are readily released in gaseous form from the liquid media under these conditions with urea and biuret conversions reaching 100%. The systems of Jones and others do not consider the presence or hydrolysis of biuret, which is present in essentially all regular commercial grades of urea, nor provide for a controllable continuous supply of gaseous ammonia and carbon dioxide for feeding to a nitrogen oxides emissions control useable by the SCR method, nor for other industrial uses requiring ammonia.
Jones also describes in his FIG. 4 a loop system for generating ammonia vapor in which the urea containing solution is hydrolyzed under high pressure conditions in order to keep the ammonia formed in solution in the catalyst chamber, until the pressure is subsequently released by flashing to a lower pressure through a pressure relief valve, but with no control of the gas pressure or gas flow. Such a system does not provide a suitable means for controlling the ammonia flow to meet the demanding control requirements of NO.sub.x control systems and other process requiring that the supply of ammonia be at a rate which essentially matches the up-take rate of the NO.sub.x or other component being reacted, nor show or teach the necessity of keeping the ammonia and carbon dioxide product vapor mixture heated for feeding to the NO.sub.x control system. The loop system described by Jones utilizes a two step system for heating and reacting, comprised of an initial heat exchanger and a following conversion catalyst chamber, both of which are under high pressure. This means that the substantial amount of heat required for carrying out the endothermic hydrolysis reaction to release ammonia must be contained in the circulating liquid stream as sensible heat prior to being brought into the catalyst reaction chamber where the endothermic reaction heat is required for the hydrolysis to proceed. Accordingly, a very large amount of liquid must be pumped up to the high operating pressure specified and the need to heat the solution to a substantially higher temperature than with the present invention, which supplies the heat required at a lower temperature directly where the endothermic reaction is occurring and does not suffer the high pumping cost for recompressing the circulating liquid, thereby avoiding both of these problems and the high costs involved for so doing.
Another consideration with the process of Jones is the matter of his catalysts, their life and the manner of their use. All of the catalyst salts or oxides indicated will tend to be decomposed or solubilized by continuous contact with hot ammonia containing liquid streams. Ammonia is a powerful solvent, particularly at high pressures and temperatures around or above its critical temperature. The actual chemical composition of the catalysts employed is not identified or specified, other than by an elemental name. Salts of vanadium, chromium and molybdenum are classified as toxic and their discharge into flue gas streams would be severely restrained from an environmental viewpoint. In their metallic form, these elements were not found to show significant catalytic activity.
Young in U.S. Pat. No. 5,252,308 describes a two-step acidic chemical process for converting urea to produce ammonia that employs polytropic acids, such as H.sub.3 PO.sub.4. The safety advantages of on-site ammonia generation are pointed out and that ammonia generated by this method can be used for the removal of nitrogen oxides from combustion gas. His two-step acid process, however, is substantially more complicated in its equipment requirements and method of operation, and is not readily adapted to meet the critical rate of ammonia generation required. The ammonia is contacted with the combustion gases at a pressure and rate consistent only with the reaction rate at the reaction temperature. The pressure during ammonia evolution is not controlled or maintained at any set elevated level. The application of the process to the removal of nitrogen oxides by the SCR type process, or for the "conditioning" of flue gas to give improved removal of fine particulate matter, is not disclosed.
Miller and Laudal in U.S. Pat. No. 5,034,030 describe a process for flue gas conditioning applied to fabric filtration in which ammonia gas and sulfur trioxide are injected for improved performance of a fabric filter. Ammonia gas and sulfur trioxide have also been used for conditioning of electrostatic precipitators as discussed in U.S. Pat. No. 4,533,364, Altman, et. al. Dismukes also describes the conditioning of fly ash with ammonia. Lookman, et. al. in U.S. Pat. No. 5,567,226 discuss the controlled injection of ammonia for improving the performance of particulate control devices.